Is there a fast way to render an obj+mtl file to PNG
We can't help you without knowing what it is in your scene that is taking the time. Two seconds is almost nothing, so presumably it is a very simple scene and rendered using the Eevee or Internal engine.

Simplest answer is to lower the resolution. Beyond that, I would suggest that this question would be better on BlenderArtists, where there is more room to discuss things like render settings and their impact on the final image.

Regardless, I suggest you post an example file here: PasteAll.org